Fix bug where a server can't be stopped an restarted without waiting …

…several seconds (#15729)

src/prefect/cli/server.py

@@ -251,6 +251,7 @@ def start(
     # check if port is already in use
     try:
         with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+            s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
             s.bind((host, port))
     except socket.error:
         if pid_file.exists():